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Building from source

Host platforms

LLVM Embedded Toolchain for Arm is built and tested on Ubuntu 18.04 LTS.

The Windows version of LLVM tools is built on Windows Server 2019 and lightly tested on Windows 10. Windows package provides runtime libraries built on Linux, because of their limited Windows support.

Building and testing on macOS is functional but experimental.

Installing prerequisites

The build requires the following software to be installed, in addition to the LLVM requirements:

  • CMake 3.20 or above
  • Meson
  • Git
  • Ninja
  • Python
  • QEMU (for running the test suite, so optional)

On a Ubuntu 18.04.5 LTS machine you can use the following commands to install the software mentioned above:

$ apt-get install python3 git make ninja-build qemu
$ apt-get install clang # If the Clang version installed by the package manager is older than 6.0.0, download a recent version from https://releases.llvm.org or build from source
$ apt-get install cmake # If the CMake version installed by the package manager is too old, download a recent version from https://cmake.org/download and add it to PATH
$ pip install meson

On macOS, you can use homebrew:

$ brew install llvm python3 git make ninja qemu cmake
$ pip install meson

Some recent targets are not supported by QEMU, for these the Arm FVP models are used instead. These models are available free-of-change but are not open-source, and come with their own licenses.

These models can be downloaded and installed (into the source tree) with the fvp/get_fvps.sh script. This is currently only available for Linux. By default, get_fvps.sh will run the installers for packages which have them, which will prompt you to agree to their licenses. Some of the packages do not have installers, instead they place their license file into the fvp/license_terms directory, which you should read before continuing.

The installer for the cryptography plugin requires a graphical display to run: it cannot run in a pure terminal session such as you might start via SSH. Also, it will prompt for a directory to install the plugin into. You should enter the pathname fvp/install relative to the root of your checkout. The installer will automatically append a subdirectory FastModelsPortfolio_11.27 to the end of that, and respond with a warning such as 'Directory [...] not found (but in patch mode). Continue installation?' Say yes to this prompt, and continue clicking 'Next' until installation is complete.

For non-interactive use (for example in CI systems), get_fvps.sh can be run with the --non-interactive option, which causes it to implicitly accept all of the EULAs and set up the correct install directories.

If you have previously downloaded and installed the FVPs outside of the source tree, you can set the -DFVP_INSTALL_DIR=... cmake option to set the path to them.

Testing with FVPs is disabled by default, but QEMU tests will still be run, and all library variants will still be built. Testing with FVPs can be enabled by setting the -DENABLE_FVP_TESTING=ON CMake option if you have installed the models as described above.

Customizing

To build additional library variants, edit the CMakeLists.txt by adding calls to the add_library_variant CMake function using existing library variant definitions as a template.

To build additional LLVM tools, edit the CMakeLists.txt by adding required tools to the LLVM_DISTRIBUTION_COMPONENTS CMake list.

Building

The toolchain can be built directly with CMake.

export CC=clang
export CXX=clang++
mkdir build
cd build
cmake .. -GNinja -DFETCHCONTENT_QUIET=OFF
ninja llvm-toolchain

To make it easy to get started, the above command checks out and patches llvm-project & picolibc Git repos automatically. If you prefer you can check out and patch the repos manually and use those. If you check out repos manually then it is your responsibility to ensure that the correct revisions are checked out - see versions.json to identify these.

export CC=clang
export CXX=clang++
mkdir repos
git -C repos clone https://github.com/llvm/llvm-project.git
git -C repos/llvm-project am -k "$PWD"/patches/llvm-project/*.patch
git -C repos clone https://github.com/picolibc/picolibc.git
git -C repos/picolibc am -k "$PWD"/patches/picolibc/*.patch
mkdir build
cd build
cmake .. -GNinja -DFETCHCONTENT_SOURCE_DIR_LLVMPROJECT=../repos/llvm-project -DFETCHCONTENT_SOURCE_DIR_PICOLIBC=../repos/picolibc
ninja llvm-toolchain

Testing the toolchain

ninja check-llvm-toolchain

Packaging the toolchain

After building, create a zip or tar.xz file as appropriate for the platform:

ninja package-llvm-toolchain

Cross-compiling the toolchain for Windows

The LLVM Embedded Toolchain for Arm can be cross-compiled to run on Windows. The compilation itself still happens on Linux. In addition to the prerequisites mentioned in the Installing prerequisites section you will also need a Mingw-w64 toolchain based on GCC 7.1.0 or above installed. For example, to install it on Ubuntu Linux use the following command:

# apt-get install mingw-w64

The MinGW build includes GCC & MinGW libraries into the package.

The following three libraries are used:

Library Project Link
libstdc++-6.dll GCC https://gcc.gnu.org
libgcc_s_seh-1.dll GCC https://gcc.gnu.org
libwinpthread-1.dll Mingw-w64 http://mingw-w64.org

The libraries are distributed under their own licenses, this needs to be taken into consideration if you decide to redistribute the built toolchain.

To enable the MinGW build, set the LLVM_TOOLCHAIN_CROSS_BUILD_MINGW option:

cmake . -DLLVM_TOOLCHAIN_CROSS_BUILD_MINGW=ON
ninja package-llvm-toolchain

The same build directory can be used for both native and MinGW toolchains.

Known limitations

  • Depending on the state of the sources, build errors may occur when the latest revisions of the llvm-project & picolibc repos are used.

Divergences from upstream

See patches directory for the current set of differences from upstream.

The patches for llvm-project are split between two folders, llvm-project and llvm-project-perf. The former are generally required for building and successfully running all tests. The patches in llvm-project-perf are optional, and designed to improve performance in certain circumstances.

To reduce divergence from upstream and potential patch conflicts, the performance patches are not applied by default, but can be enabled for an automatic checkout with the APPLY_LLVM_PERFORMANCE_PATCHES option.

Building individual library variants

When working on library code, it may be useful to build a library variant without having to rebuild the entire toolchain.

Each variant is built using the arm-runtimes sub-project, and can be configured and built directly if you provide a path to a LLVM build or install.

The default CMake arguments to build a particular variant are stored in a JSON format in the arm-multilib/json/variants folder, which can be loaded at configuration with the -DVARIANT_JSON setting. Any additional options provided on the command line will override values from he JSON. -DC_LIBRARY will be required to set which library to build, and -DLLVM_BINARY_DIR should point to the top-level directory of a build or install of LLVM.

(The actual binaries, such as clang, are expected to be in $LLVM_BINARY_DIR/bin, not $LLVM_BINARY_DIR itself. For example, if you're using the results of a full build of this toolchain itself in another directory, then you should set LLVM_BINARY_DIR to point at the llvm subdirectory of the previous build tree, not the llvm/bin subdirectory.)

For example, to build the armv7a_soft_nofp variant using picolibc, using an existing LLVM build and source checkouts:

cd LLVM-embedded-toolchain-for-Arm
mkdir build-lib
cd build-lib
cmake ../arm-runtimes -G Ninja \
  -DVARIANT_JSON=../arm-multilib/json/variants/armv7a_soft_nofp.json \
  -DC_LIBRARY=picolibc \
  -DLLVM_BINARY_DIR=/path/to/llvm \
  -DFETCHCONTENT_SOURCE_DIR_LLVMPROJECT=/path/to/llvm-project \
  -DFETCHCONTENT_SOURCE_DIR_PICOLIBC=/path/to/picolibc
ninja

If enabled and the required test executor available, tests can be run with using specific test targets: ninja check-picolibc ninja check-compiler-rt ninja check-cxx ninja check-cxxabi ninja check-unwind

Alternatively, ninja check-all runs all enabled tests.

Building sets of libraries

As well as individual libraries, it is also possible to build a set of libraries without rebuilding the entire toolchain. The arm-multilib sub-project builds and collects multiple libraries, and generates a multilib.yaml file to map compile flags to variants.

The arm-multilib/multilib.json file defines which variants are built and their order in the mapping. This can be used to configure the project directly

For example, building the picolibc variants using an existing LLVM build and source checkouts:

cd LLVM-embedded-toolchain-for-Arm
mkdir build-multilib
cd build-multilib
cmake ../arm-multilib -G Ninja \
  -DMULTILIB_JSON=../arm-multilib/json/multilib.json \
  -DC_LIBRARY=picolibc \
  -DLLVM_BINARY_DIR=/path/to/llvm \
  -DFETCHCONTENT_SOURCE_DIR_LLVMPROJECT=/path/to/llvm-project \
  -DFETCHCONTENT_SOURCE_DIR_PICOLIBC=/path/to/picolibc
ninja

To only build a subset of the variants defined in the JSON file, the -DENABLE_VARIANTS option controls which variants to build. E.g, -DENABLE_VARIANTS="aarch64a;armv7a_soft_nofp" only builds the two variants of aarch64a and armv7a_soft_nofp.

If enabled and the required test executor available, tests can be run with using specific test targets:

ninja check-picolibc ninja check-compiler-rt ninja check-cxx ninja check-cxxabi ninja check-unwind

Alternatively, ninja check-all runs all enabled tests. ninja check-<VARAINT_NAME> runs all the tests for that specific variant.